1. Introduction
This invention relates to data communication systems and more particularly to data communication system adapted for use on power lines for monitoring loads, providing load control, detecting and locating faults, providing transformer protection, automatic meter reading and two-way communications and the like.
2. Related Applications
This application relates to application serial numbers 529,998, 530,000 and 530,129 filed concurrently herewith by the present applicant and assigned to the assignee of the present invention.
3. Field of the Invention
Electrical power to individual residences and industrial users is generally provided in a local service area by a power company which receives its power from either a local generating station, a high voltage transmission tie-line, or both. The power company then distributes this power through a power distribution network originating from the power source and terminating at the user's location. Simplified, this distribution network comprises a tree-like structure that branches out from the source of power through several levels of distribution voltages and ultimately to a large number of users. (It is understood that power companies frequently provide additional interconnections at various voltage levels to form loops for improving the reliability of the service. For purposes of description, this complexity is ignored since it does not affect the operation of the present invention.) The generated power is distributed by step-down and distribution transformers. The transformers may be considered the nodes with each node receiving its power at a higher voltage level and delivering its power to one or more branches at a lower voltage level.
The distribution of electrical power in this manner requires a high degree of reliability to insure continuous service to the power users. The power company is therefore under a heavy burden of systematically monitoring the various loads, providing load control where necessary, detecting the existence of power outages and location of faults producing such outages. While some degree of automation has been utilized to meet these needs of the power company, monitoring and control of loads, as well as fault detection and numerous other functions encountered in power distribution are still primarily handled by company personnel assigned to perform these various tasks. Additionally, since power users are billed only for the power consumed, it is presently necessary to employ personnel for reading each of the watt-hour meters at the various customer locations. The performance of these various functions therefore necessitates a large number of specially skilled individuals. The costs of maintaining such skilled individuals is ultimately passed on to the consumer, thereby increasing his costs.
4. Prior Art
In the past, many attempts have been made to automate some or all of these functions and, possibly, to provide a two-way communication system as well. One possibility that has been considered is the use of telephone lines or other communication links independent of the power line distribution system. However, this approach is too costly for the benefits received. Alternatively, systems have been proposed utilizing the power line distribution system itself. These systems have proven unsatisfactory for several reasons. One reason is the noise on the line; for example, from motors, atmospheric noise (the line acts as an antenna), switching of loads, static, arcs, etc. Another reason is the impedance variations in the line itself, e.g., due to humidity changes, load variations, variations in height above ground and the like, as well as data transmission barriers in the form of impedance discontinuities caused by step-down and distribution transformers, and series and shunt capacitors to control power factor.
These and other difficulties combine to produce what may be considered a uniquely hostile environment in which to reliably transmit data. The result is that any automation of the meter reading function was usually limited to only one level in the power distribution system, e.g., from the pole repeater to the consumer, with other communication links to a central office. Further, two-way communication has been limited due to the number of transmission errors that may occur with systems of the prior art. Similarly, power line transmission systems of the prior art do not provide random access to the user locations.